A conventional OFDM signal receiver device (hereinafter also simply referred to as a “receiver device”) performs a Fourier transform on an OFDM signal. Then, in synchronization with a subcarrier component obtained as a result of the Fourier transform, the receiver device generates a pre-transmission pilot signal (hereinafter also referred to as a “transmission pilot signal”), which is a known signal. By dividing a subcarrier component of a pilot signal in the subcarrier component contained in a received OFDM signal (hereinafter the pilot signal is also referred to as a “received pilot signal”) by the transmission pilot signal, the receiver device calculates transmission channel characteristic corresponding to the received pilot signal. Then, by filtering the transmission channel characteristic of the received pilot signal with an interpolation filter for interpolating the transmission channel characteristic along a time axis and a frequency axis, it calculates transmission channel characteristics corresponding to all the subcarrier components. Further, by dividing the subcarrier components by the transmission channel characteristics corresponding to the output of the interpolation filter, the phase and amplitude of the subcarrier components are corrected to demodulate the subcarrier components.
Next, when convolutionally coded data are transmitted in the form of orthogonal frequency division multiplexing, the data need to be decoded by a Viterbi decoder after they are demodulated into subcarrier components. Herein, Viterbi decoding refers to a decoding method in which maximum likelihood decoding is efficiently executed utilizing the repetitive structure of convolutional code. First, the Viterbi decoder obtains a branch metric indicating the likelihood between a receiving point constellation of a subcarrier component after the phase and amplitude of which have been corrected and a signal point constellation uniquely determined depending on the modulation format. Then, it obtains all the surviving paths in possible trellises, obtains the accumulated totals of the branch metrics of the respective paths, and selects the path with the least accumulated total. The Viterbi decoder outputs the selected path state as the decoding result to reproduce the transmission data.
Japanese Patent Application Laid-Open No. 2002-344414 (hereinafter also referred to as “Patent Document 1”) shows one example of an OFDM signal receiver device having a Viterbi decoder. The OFDM signal receiver device in Patent Document 1 comprises an equalizer for waveform equalizing an amplitude modulation signal obtained by a Fourier transform and a transmission channel decoding circuit having a Viterbi decoder therein. The receiver device gives a less weight to a branch metric corresponding to a signal modulated into a subcarrier located at a band edge of the OFDM signal symbol than the weight given to a branch metric corresponding to a signal modulated into a subcarrier at the band center of the symbol. Thereby, the signal modulated into the subcarrier positioned at the band edge of the symbol has a lower degree of contribution to the state metric than the signal modulated into the subcarrier positioned at the band center.
Such a conventional OFDM signal receiver device as described above computes a Euclidean distance, which is the likelihood between a signal constellation of a demodulated subcarrier component and a signal constellation uniquely determined depending on the modulation format, and based the result, it calculates a branch metric. Therefore, the branch metric obtained by the conventional receiver device does not take into consideration the electric power corresponding to the average of the noise components contained in demodulated signal (hereinafter also referred to as “average noise power”) or the electric power ratio (hereinafter also referred to as “signal-power-to-noise-power ratio”) of desired signal power (for example, electric power corresponding to received signal) to noise power, although the Euclidean distance (hereinafter also simply referred to as a “distance”) between the signal constellations is taken into consideration. In addition, with the receiver device of Patent Document 1, although branch metrics are calculated taking the positions of the symbols within the band into consideration, the average noise power corresponding to the noise component contained in the demodulated signal or the signal-to-noise power ratio is not taken into consideration.
Nevertheless, when receiving OFDM signal while in transit, the power of received signal changes greatly over time, and therefore, the average noise power or the signal-to-noise power ratio also varies over time. However, the Euclidean distance that is calculated from demodulated signal does not take into account the absolute amount of the average noise power contained in the demodulated signal or the signal-to-noise power ratio; thus, there has been a problem that the adverse effect caused by the power corresponding to the noise cannot be suppressed in decoding the demodulated signal and the error rate corresponding to the signal after decoding the demodulated signal cannot be made sufficiently small.